Terephthalic acid is produced by liquid-phase oxidation reaction of a p-phenylene compound such as p-dialkyl benzene, typically, p-xylene. In this reaction, a heavy metal catalyst such as cobalt and manganese, or the catalyst to which a promoter such as a bromine compound and an aldehyde is further added is usually used in the presence of an acetic acid solvent.
A terephthalic acid-containing slurry obtained from such a liquid-phase oxidation reaction is usually subjected to crystallization to reduce its temperature and then subjected to solid-liquid separation under a pressure close to normal pressures to thereby obtain a cake of terephthalic acid.
On the other hand, an oxidation reaction mother liquor obtained in the solid-liquid separation contains useful catalyst components derived from the catalyst such as heavy metal ions and bromide ions. When industrially practicing the above reaction, it is necessary to recycle and reuse these catalyst components and thereby reduce production costs.
The simplest recycling method is a method in which the oxidation reaction mother liquor is fed as itself back to a reaction system and reused therein (mother liquor recycling), and this method has been extensively used in commercial production processes. However, the oxidation reaction mother liquor contains various organic impurities by-produced in the liquid-phase oxidation reaction or inorganic impurities produced owing to corrosion of an apparatus used. If the oxidation reaction mother liquor is reused as itself in the reaction system, the concentration of these impurities in the reaction system tends to be gradually increased. As a result, it has been confirmed that when the concentration of the impurities exceeds a predetermined level, the liquid-phase oxidation reaction tends to be adversely affected.
For example, it has been reported that in the case of terephthalic acid, the proportion of the oxidation reaction mother liquor fed back to the reaction system (mother liquor recycling rate) is usually from 70 to 98%. The remaining oxidation reaction mother liquor which does not serve for reuse in the reaction system and is present in an amount of from 2 to 30% is fed to a step of recovering acetic acid as a solvent (the mother liquor not fed back to the reaction system is called a “purge mother liquor”).
As a method of recovering the catalyst components from the oxidation reaction mother liquor fed to the acetic acid recovering step and reusing it, there have been proposed the method using an anion-exchange resin (Patent Documents 1 to 4) and the method using a pyridine ring-containing chelate resin (refer to Patent Documents 5 and 6).
In Patent Document 2, it is described that an oxidation reaction mother liquor contains fine aromatic carboxylic acid crystals leaked from an aromatic carboxylic acid slurry upon solid-liquid separation thereof or precipitated by temperature drop of the oxidation reaction mother liquor, and therefore when continuously feeding the oxidation reaction mother liquor to a resin column to contact with a resin, it is necessary to remove the fine crystals using a filter, etc., in order to prevent deposition of the fine crystals on an upper portion or an inside portion of a resin layer. However, Patent Document 2 fails to describe a specific method for removing the fine crystals. In Patent Document 3, it is described that solids need to be removed by filtering the oxidation reaction mother liquor in advance at a temperature not higher than a temperature of the operation. Although an installation place of a high-grade filter for removing the fine crystals is specified in a schematic flow-diagram of the process, details of the filter itself is not described. In Patent Documents 5 and 6, it is described that an oxidation reaction slurry is preferably subjected to solid-liquid separation such that a content of crystals in the oxidation reaction mother liquor is 0.1% or less, but a filter for removing the fine crystals is not described.
In Patent Document 4, it is described that a mother liquor purge flow is filtered through a filter medium to recover and recycle an insoluble aromatic carboxylic acid and the other insoluble components. As examples of the filter medium, there are mentioned a microfiltration filter medium, an ultrafiltration filter medium, a membrane filter medium, a cross-flow filter medium, a hydro-cyclone filter medium, a cross-flow ceramic microfiltration filter medium, a bag filter medium, a sintered metal cross-flow ceramic microfiltration filter medium, a cross-flow microfiltration filter medium or the like. It is also described that among these filter media the cross-flow filtration using an anticorrosive and high temperature-resistant ceramic filter is preferred because the aromatic carboxylic acid as a product material trapped on the filter medium can be continuously removed and recovered. However, details of the filtering operation are not disclosed, and it is merely described that a suitable turbulence is obtained when a Reynolds number of a fluid entering into a flow path of the ceramic filter is larger than about 13,000.
When the oxidation reaction mother liquor containing fine crystals is processed in a cross-flow filtration using a ceramic filter, the fine crystals deposited on a filtering membrane of the ceramic filter are filtered while always washing out with a circulating fluid flowing through a flow path to obtain a clear filtrate. However, since a filtering performance is gradually deteriorated owing to the deposition of the fine crystals, it is required to interrupt the filtering operation to clean the ceramic filter.
As the method of the cleaning, there may be used a method in which the cleaning is conducted by interrupting circulation of the oxidation reaction mother liquor flowing through the flow path of the ceramic filter, or a method in which the cleaning is conducted while continuing the circulation thereof.
In the method in which the circulation is interrupted, in order to remove the deposited fine crystals, a wash solvent capable of dissolving the fine crystals (specifically, for example, an acetic acid solvent) flows through the flow path of the ceramic filter to clean a surface of the filtering membrane, and the wash solvent flows from the flow path side to the filtration side to penetrate through the filtering membrane (normal washing with the wash solvent). In addition, the wash solvent is also allowed to penetrate from the filtration side to the flow path side of the ceramic filter (back washing with the wash solvent). Thus, by using the method in which the circulation is interrupted, it is possible to fully clean the ceramic filter and completely restore a filtering performance thereof (refer to Patent Documents 7 and 8). However, the method in which the circulation is interrupted has various problems such as need of using a large amount of the wash solvent, time-consuming procedure, complicated valve operations upon feed and interruption of the oxidation reaction mother liquor as well as upon feed and interruption of the wash solvent, occurrence of treatment of a large amount of the used wash solvent, and so on.
In the method of cleaning the ceramic filter in which the circulation of the oxidation reaction mother liquor is not interrupted, a filtrate is allowed to penetrate from the filtration side to the flow path side of the ceramic filter (back washing with the filtrate). This method requires no fresh wash solvent (producing no used wash solvent) and can be easily conducted only by interrupting the filtering operation, so that a filtering performance of the ceramic filter can be substantially restored. Therefore, the above method has been frequently employed in filtering operations by a ceramic filter in a general cross-flow type (refer to Patent Documents 9 and 10).
In Patent Document 9, it is described that while circulating a stock solution, a pressure higher than that on the stock solution side is instantaneously applied onto a filtrate in a filter container by a piston, and the filtrate is forced into the filter at a linear velocity larger than the filtering linear velocity to conduct the back washing operation of the filter. This method is characterized in that the back washing time is very short.
From the contents of Examples in this document, it is construed that the feature of the method resides in that when the filtering linear velocity is lowered owing to clogging of the filter, the back washing operation with the filtrate is conducted at a linear velocity larger than such a lowered filtering linear velocity.
However, the above method has posed problems such as use of the piston facility when industrially practiced. Further, the differential pressure applied upon the back washing operation is not described.
In claim 7 of Patent Document 10, it is described that a filtrate is reversely flowed from an outer periphery side to a flow path side of a filter to remove particles fixed on a surface of a filtering membrane. In addition, in the specification, it is described that upon conducting the cross-flow filtration, a periodic back washing is preferably performed in order to prevent fixing of a cake layer on the surface of the filtering membrane. There is mentioned such a back washing mechanism that a sump for back washing (back washing pot) is provided on a recovery side of the filtrate, and the filtrate is reversely flowed toward the flow path side by an air compressor or a pump. Further, in Example, as shown in FIG. 1, there is illustrated a structure including the air compressor and the back washing pot, which is capable of reversely flowing the filtrate from the outer periphery side of the filter to the flow path side thereof.
However, any of these Patent Documents merely describe a general back washing, but fail to describe requirements for efficiently conducting the back washing operation. Also, as the back washing with the filtrate is repeated, a recovery rate of a filtering performance is gradually lowered so that the filter finally fails to exhibit a desired filtering performance. In such a case, after the circulation of the oxidation reaction mother liquor is interrupted, the ceramic filter should be cleaned by normal washing with the wash solvent and/or back washing with the wash solvent.